http://www.reedbeta.com/blog/2013/01/12/quick-and-easy-gpu-random-numbers-in-d3d11/
https://github.com/openglsuperbible/sb6code/blob/master/bin/media/shaders/ssao/ssao.fs.glsl http://math.stackexchange.com/questions/337782/pseudo-random-number-generation-on-the-gpu
http://www.trentreed.net/blog/screen-space-ambient-occlusion/